Modern Intermediate Calibers 001: The 5.56x45mm

Various 5.56mm rounds, left to right: 55gr M193, 55gr French ball, M855 (made in Korea), Mk. 262 Mod. 1, Mk. 318, M855A1

“Nathaniel’s starting another series? Oh brother…”

Bear with me. I think that there is a lot of rhetoric thrown around with regards to modern defensive/military calibers that exists apart from the necessary additional data and context of the respective ammunition types being discussed. Therefore, I’d like to take some time to share my perspective on modern calibers relevant to the military infantry caliber question with TFB’s readership.

First, we’ll tackle the familiar 5.56mm round, a venerable but also controversial caliber designed in the late 1950s for the AR-15 rifle. It’s by far the most popular “black rifle” caliber in the US today, as well as the standard caliber for NATO, the USA, and her allies. A longer, comprehensive history of this caliber will follow later, but today we’ll take a look at the 5.56mm caliber as it exists now, and its ballistic characteristics.

5.56mm is what is known as “small caliber, high velocity”, meaning that it shoots a small-diameter, lightweight bullet at very high muzzle velocities compared to other rifle cartridges. Despite its small size, the standard-weight 5.56mm produces more velocity even from carbine barrels than some of the fastest .30-06 loads do from longer hunting rifle barrels. Let’s take a look at the ballistics of two different 5.56mm loads from two different barrel lengths – 14.5″ and 20″:

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Here we can see how the heavier 77gr ammunition pays dividends at longer ranges, performing better ballistically-speaking from both the 14.5″ and 20″ barrels, versus M855 from a comparable barrel length. it should be noted that M855 is ballistically similar to several other 5.56 rounds with similar bullet weight, including M855A1 and Mk. 318, and so even though it is dated technology at this time, it is still representative of the performance of other 5.56mm rounds.

Ballistics aren’t the only thing that matters, though; weight does as well. Here the 5.56mm scores very well, with total cartridge weight ranging from 11 grams (170 grains) per shot for 55gr steel-cased ammunition, to just under 13 grams (201 grains) for heavy ball loads like the 77gr Mk. 262 shown in the charts above.

Stay tuned; next time we’ll take a look at the venerable Soviet 7.62x39mm caliber!

Note: All ballistic calculations are done with JBM’s Trajectory calculator, using the ballistic coefficient appropriate to the projectile being modeled, and assuming an AR-15 as a firing platform. Also, keep in mind that there is no single true velocity for a given round; velocity can vary due to a large number of factors, including ambient temperature and chamber dimensions. Instead, I try to use nominal velocity figures that are representative of the capability of the round in question.

Nathaniel F

Nathaniel is a history enthusiast and firearms hobbyist whose primary interest lies in military small arms technological developments beginning with the smokeless powder era. In addition to contributing to The Firearm Blog, he runs 196,800 Revolutions Per Minute, a blog devoted to modern small arms design and theory. He is also the author of the original web serial Heartblood, which is being updated and edited regularly. He can be reached via email at


  • Darkpr0

    Every time I see the 5.56 bashing go down, I wish I had free copies of The Black Rifle to hand out. It should be required reading for anyone posting about ballistics in comment threads on certain firearms-related websites.

    • Kivaari

      Ezell’s “Great Rifle Controversy” is good as well.

  • 624A24

    Nat, have you considered compiling your articles onto a Wikia site? These articles are extremely informative in technical and engineering aspects, and many stuff in each article could be linked to other articles.

    • I’m really glad people like this stuff. I started doing these short little technical posts as a way to get away from press releases for new AR-15 dinguses, heh. It’s very good to hear that people are getting something out of it.

      • Grzegorz Widera

        Could you consider to add speed of sound line to velocity and energy graphs in the future. It helps to understood sniping applications better.

        • Good suggestion. I’ll incorporate that in the wrap-up post where I compare everything against each other.

  • Vitsaus

    Not crapping on your information as it is always quite exhaustive and generally accurate, but I’ve always understood that 5.56 and 5.45, 4.7×33, etc.. were “micro” rather than true intermediate rounds. 7.62×30, .280 British tend to be regarded as the quintessential modern intermediate rounds.

    • I don’t know where you’ve gotten that impression, but 5.56mm is definitely an intermediate round by the normal definition.

      • mig1nc

        Yes, I think it’s worth mentioning that “intermediate” refers to between a pistol caliber SMG and a full power battle rifle. Not between big rifle and little rifle. The German Stgw.44 for example introduced a cartridge between the MP40 and K98.

        • This has been clouded by proponents of intermediate calibers larger than 5.56, partly because of abbreviating “a caliber in between 5.56 and 7.62”, and partly, I suspect, out of a desire to demote 5.56.

          • ostiariusalpha

            I believe I’ve seen Max Popenker mention before that micro calibers are less than 5mm. I wonder what his citation for was?

          • I am not sure; but that is generally accepted to be the case.

  • roguetechie

    While 5.56 is a very impressive round in it’s current guise, I have to admit that I like 5.45×39 more. This is mostly because of the longer bullet, and it being designed specifically as a 500 meter round from 16.25 inch barrels.

    I’ve also put quite a bit of thought and research into the 5.56 FABRL cartridge as well as the 7.92×40/41 Cetme. Honestly, I believe that these configurations should be looked at very hard for future designs.

    • I wrote this, in my love letter to the AK:

      “The 5.45mm is a ballistician’s delight

      “If 5.56mm represents the first generation military service small caliber high velocity round, then 5.45mm represents the second. The configuration of the 5.45mm is decidedly superior to the 5.56mm; despite having a lighter bullet moving at lower velocity, the 5.45mm’s better ballistic shape allows it to perform better at range, with more energy and velocity, and less drop and drift than the 5.56mm 62gr projectile when fired from comparable rifles. This means comparable ballistic results with a lighter weight and lower-recoil round, and that’s definitely something worth appreciating.

      “While the 5.56mm currently has the better bullet selection in the Western consumer market, it’s easy for me to appreciate the “ballistician’s delight” that is the 5.45x39mm Soviet.”

      • roguetechie

        That’s exactly what I think too. Now, I tend to speculate much further afield than you’re willing to. But what do you think about maybe a 5.56 FABRL wrapped in a cbj-ms sabot?

        I’ve always wondered if we could gain the performance we want and need while also providing the, also desperately needed, weight reduction.

        This is a tempting avenue of research also because the rounds could be very gentle on aging small arms fleets due to the lower peak pressures.

        • We have better bullets now than the FABRL, although maybe not better bullet shapes.

          I’ve been harping on the idea of a low drag 5.56mm as a lighter alternative to other intermediate concepts for a while now. It’s definitely something to explore, at least.

          • roguetechie

            Yeah I agree, it definitely needs investigation. When I mention the FABRL round, and wanting to see research along those lines, I am really advocating for marrying what was learned then with what we’ve learned since then.

            Additionally though, the phenomenal reduction in time of flight at ranges beyond 300 meters that FABRL cartridges made possible, especially with lighter thermal loading smaller case capacities and lower peak pressures, really caught my attention!

            Like most people, my fascination with ultra high muzzle velocity uber alles has faded as I’ve gained more knowledge. That being said, my interest in short flight time to long range has become a near holy grail in my eyes.

            My reading of some of the research and experimental design evaluations that lead us down the SCHV SPIW ACR path has strongly indicated to me that hit probability at range can be increased most economically through the factors below in no particular order.

            Ergonomics and recoil management

            Basic and advanced sight systems

            Flat trajectory and short time of flight

          • I found this while looking through my SolidWorks files today, though you might get a kick out of it:


            Note the date. 🙂

          • roguetechie

            I always figured that we were on the same page on quite a few things…

            I just dream bigger, and I still hold out hope that someone will really push the technology to give people the opportunity to see just how amazingly effective smaller calibers can be.

          • Erm, well, I’ve dreamed up some pretty weird stuff, I just don’t normally write about it.

          • roguetechie

            Yeah, I get that. You’ve parlayed your knowledge into a very successful career based around expunging the omnipresent FUD from the gun world. I, on the other hand, am free to speculate and dream.

            Incidentally, how do you feel about cbj style sabot technology mixed with VLD SCHV projectiles as a way to wring out more performance without increased action size and as a potential way to manage thermal loads?

            I know it’s not a free ljust but I see it as a potential way to keep ammunition weight down while increasing performance out to longer distances.

            The more I’ve looked at the small arms situation the more I agree with you that we don’t need 1200 meter rounds for every rifle. Once I realized that, I started seriously looking at what we DO need. Briefly that generated a few interrelated conclusions.

            1. More consistent performance across the effective range of a weapon.

            2. Less time of flight and very flat trajectory for the limited engagements at or beyond normal ranges

            2a. Fire fights involve fleeting targets at highly variable range. With mechanization and artillery it behooves us to shoot and scoot.

            2b. People and vehicles maneuver quickly and violently in these encounters. Because of this, a round that takes less time to arrive makes for easier determination of aim point when leading a target.

            3. Weight! With all of these conditions singularly or combined, it leads one to conclude that we really cannot afford to keep the weight per unit of ammunition we currently have! We’re in a position that basically demands reduced weight ammunition, while also not sacrificing range etc.

            4. Lead… No I’m not a rabid tree fondler. We just don’t really have viable or quick to restart production assets to produce militarily significant amounts of lead. We do however have large reserves of petroleum and at least a decent amount of capacity to process said petroleum into polymers. This is quite literally our only truly workable option should we lose access to Chinese production capabilities in a conflict.

            Also speaking selfishly, we’re way more likely to get access to a FABRL type round than any CT round they decide to implement.

          • What do you mean by “CBJ Sabot Technology”? I know what the 6.5x25mm CBJ is, I just wasn’t aware they had any proprietary specialness to their sabots.

          • roguetechie

            Their sabots are actually very neat and in my opinion well thought out. They actually have a patent on the sabot design registered under bertil johannsen.

            Basically there’s the polymer part we’ve all seen and then a cup that is metallic and envelopes probably 10-20% of the sabot base. The patent shows 4 triangular shape tabs cut radially in the sabot base bent upwards into the cup. This allows them to use prescored but uncut sabot petals because the mechanical force of the sabot squeezing into the rifling combined with the radial tabs actually forces it to separate along the scoring when it pops free as it leaves the muzzle.

            I’m guessing that this is how they so successfully launch their light projectiles in a consistent enough manner to allow 300 meter hits on the large target using an open bolt SMG.

            I also suspect that it has potential to scale nicely to assault rifle or even battle rifle size. So do the owners of CBJTECH AB who are currently looking for partners to work with on scaling up the technology. Hopefully someone in the industry takes up the torch to run with this project, because I’m pretty sure it will pay out in a big way.

          • Oh, interesting, I did not know that!

      • Xtorin O’hern

        i just wonder why nobody has taken 5.56x45mm cases and necked them down to 5.45, wouldn’t the extra velocity and low recoil make for a pretty awesome round?

        • 5.56mm has a max overall cartridge length of 2.26″, or 57.4mm, and a case length of 1.76″ or 44.7mm. That gives it a bullet space of exactly half an inch, 12.7mm.

          5.45x39mm has a max overall cartridge length of 57mm, or 2.244″, and a case length of 39.8mm, or 1.567″. Bullet space is 17.2mm, 0.677″.

          So that means 5.45x39mm bullets won’t fit within the overall length of a 5.56mm round, preventing them from feeding in magazines. Since 5.45mm bullets are only three and a half thousandths of an inch smaller than 5.56mm projectiles, there wouldn’t be much point to the conversion.

  • Risky

    Could you explain to me why the in first chart for velocity all the rounds seem to stabilize identically after they go subsonic? It’s a pretty striking visualization on that chart that has to have some interesting science behind it.

    • The short answer is that drag in the subsonic flight regime is dramatically reduced, therefore the projectiles all lose velocity at a much lower rate than they do in supersonic flight. This is reflected by the “elbow” on graphs of JBM data, and the fact that JBM accounts for this (and the majority of other ballistic programs do not) is one of the major reasons I use it.

      If you want to understand this topic in greater depth, check out the series of videos by Shell Oil from the 1950s describing subsonic, transonic, and supersonic flight. Here’s the transonic one:

  • Mark K.

    Once again the data is there- the full 20″ lenght barrel is where this round shines.

    • Every round in the world gives better ballistics from a longer barrel, at least until you cross the friction threshold…